Add body to curl DELETE request if one is specified.

The header will be there. Fixes issue #6315 (https://github.com/zendframework/zf2/issues/6315).

When providing a request body for a DELETE request, Zend\Http\Client will add the 'Content-Length' header but will not add and send the actual body resulting in a curl call that hangs for a very long period of time. The client should send both a body and the header since DELETE requests can have a body. From the IETF spec: "The presence of a message-body in a request is signaled by the inclusion of a Content-Length or Transfer-Encoding header field in the request's message-headers." -- http://tools.ietf.org/html/rfc2616